The structure of fasteners, and particularly screws, is a highly developed art. For many years screws have been formed with specific thread formations that have permitted them to better perform specific tasks. Thus, screws have been developed for the specific purpose of securing gypsum wallboard to wooden studs, such fasteners being known as dry wall screws, and such screws possess threads of varying convolutions. As an example, U.S. Pat. No. 3,703,843 to Laverty describes such a screw said to have increased holding power. The screw is formed with a thread extending from the screw head to the tapered point. A second thread extends uniformly along the fastener shank between the convolutions of the first thread from the point to a location less than one-half of the distance from the point to the head. According to Laverty, the multiple threads facilitate entry into the material being pierced, while the later encountered single thread eases passage of the screw into the material once initial entry has been accomplished. In other prior art threaded fasteners are disclosed in which the threads have different pitches and even different directions of spiraling.
In the use of many threaded fasteners, either the fastener, itself, forms an orifice in the objects being joined, in which case the screw is said to be self-drilling, or the fastener shank is positioned in a preformed hole that extends through the objects; In either instance, unless some means is provided for sealing against ingress of the elements, and specifically water, into the orifice through which the fastener shank passes, water will enter the hole and cause corrosion or galvanic activity, either of which is highly deleterious to the stability of the juncture over time. Thus, in an effort to seal the orifice from the elements, it has become commonplace to provide a washer or gasket at the head of the screw, and generally the inner surface of the screw head is shaped to present a surface against which the washer seats to effect a seal between the screw head and the surfaces of the structures being joined.
As the use of a washer to underlie the head of each fastener has become prosaic, it is also commonplace for fasteners to be shipped to the building site with each screw having a washer loosely attached to it, thereby saving the time of a workman having to extract fasteners and washers from separate containers and thread the shank of each screw through a washer before forming an aperture with the fastener if the screw is self-drilling. As with many labor intensive operations in erecting a building, utilizing pre-threaded fasteners can effect significant temporal savings.
Nevertheless, the problem has then arisen as to how to maintain a rubber washer that is loose fitting and is not attached to the screw in position on the screw shank adjoining or contiguous with the head of the screw so that, despite normal handling both in shipment and in the field, the washer will be retained in position adjacent to the screw head so that the screw can be drilled in place with minimum effort and with the washer positioned to effect its sealing function.
It is, therefore, a primary object of the present invention to provide a metal fastener, namely, a screw of such configuration that a washer will be maintained at least loosely in place at the screw head in normal handling of the screw and washer during shipment and at the job site.
It is a further object of my invention to provide the combination of a washer and a screw in which the washer is threaded onto the shank of the screw to a position adjacent the screw head and maintained in that position despite moderate jostling of the screw-washer combination.